1. Field of the Invention
The group of inventions that is the substance hereof concerns the designs and adjustment of aberrometers, ophthalmologic instruments used in medical clinical practice for measuring aberrations of the human eye. The aberrometer presented herein as an invention is intended for automatically measuring aberrations of the human eye and determining subjective visual acuity while, simultaneously, selecting the best spherocylindrical correction in various accommodation statuses of the eye and, in the second modification of the instrument, for researching the influences of higher-order aberrations on subjective visual acuity and making forecasts as to the results of correcting vision with eyeglasses, individual contact lenses, intraocular lenses or laser ablation.
The alignment system of the aberrometer intended for fine-adjusting the distance between the eye and the instrument (the alignment of the entrance pupil of the gage with the pupil of the eye) is its integral part yet may be used, according to its function, with any ophthalmologic instruments used, for instance, in clinics while operating on the human eye, diagnosing its pathological conditions or measuring its characteristics.
The method implemented in the alignment system of the aberrometer may be used for adjusting any ophthalmologic instrument.
2. Description of the Related Art
In clinical practice, visual acuity is determined with the use of tables containing symbols or pictures whose sizes, normally, at a distance of 5 meters, correspond to various angular sizes of their images on the eye retina. The angular size of one minute corresponds to 100% or 1 vision (20/20 in English language literature). A tested person estimates the smallest size of the symbols he or she can discern, which is the indication of his or her visual acuity. For instance, if the size of still discernible symbols corresponds to the angle of 5 minutes, the visual acuity is 50% (0.5). The best spherocylindrical correction is determined using exchangeable test lenses (phoroptors) while continuously checking the results against the test table. The process is rather tedious and lengthy, especially in the case of complicated astigmatism (see E. I. Kovalevsky, Ophthalmology, Moscow, Medicine publishers, 1995, pp. 45-83). The selection of correction lenses may be accelerated through various methods of measuring refraction such as retinoscopy or the use of automatic refractometers. These instruments indicate the initial parameters of test corrective lenses. Aberrometers are more advanced instruments measuring the optical characteristics of the eye. They not only measure refraction and astigmatism but also higher-order aberrations.
There is a device with the same function as the invention that is the substance of this application (the Aberrometer with a System for Testing Visual Acuity). It is an ophthalmologic instrument described in Objective Measurement of Wave Aberrations of the Human Eye with the Use of a Hartmann-Shack Wave-front Sensor by Junzhong Liang, Bernhard Grimm, Stefan Goelz and Josef F. Bille (JOSA A, Volume 11, Issue 7, July 1949, 1994) intended for measuring aberrations of the human eye. It contains a point light source. When projected on the retina, its light creates a virtual reference source whose radiation is diffracted by the retina and, while passing through the optical systems of the eye, becomes phase-modulated in accordance with the combined optical aberrations of it, then goes through the system measuring the shape of the wavefront of the light leaving the eye. The system is a Shack-Hartman's sensor whose output signal goes to the instrument's controls, including a computer that processes the data, restores the aberration chart, stores the data and controls the instrument following its operator's commands. We used this device as a prototype because the sum total of its significant characteristics makes it the closest to the one that is the substance of this application—in its both modifications.
While measuring aberrations, an increased dynamic range, that is, the best ratio between the maximal and minimal aberration values may not be achieved with the use in the device of the Shack-Hartman's wavefront sensor because it does not allow measuring aberrations with large and small amplitudes equally well. If the parameters of the sensor are selected so as to increase the range of measurement, small aberrations are measured with considerable errors. Because both small and large aberrations occur in clinical practice, this factor is a serious disadvantage of aberrometers equipped with such sensors.
There are various methods of increasing the dynamic range of measurement of wavefront sensors.
For instance, there is a technical solution (U.S. Pat. No. 6,550,917) suggesting the use of a special pre-compensation reference beam creating a small-diameter light spot on the retina. When this is done, the image of the reference beam in the focus of the micro-lens matrix will also be small. Yet when the light spot on the retina is small, the influence of phase and amplitude speckle modulation increases, which results in a lower precision of measurements. Besides, when a probing beam of a considerable diameter is used, the conditions requiring a single-pass measurement are not met and certain aberrations self-compensate during the second pass of the beam. When the diameter of the probing beam is small, between 0.5 mm and 0.8 mm, the size of the light spot on the retina is practically independent of the aberrations of the eye and the system of pre-compensation becomes mostly redundant. However, refraction compensation may be used in cases of large refraction errors (>10 D). Such system is necessary for the light beam coming out of the eye because its diameter may be as large as 8 mm.
The same invention (U.S. Pat. No. 6,550,917) suggests that the system of pre-compensation placed between an optical projection system and the eye include a device adding cylindrical correction to the probing beam. In this case, it is also suggested that the diameter of the light spot on the retina be minimal. It is believed that this increases the precision of determining the coordinates of spots, because the spots in the focus of the micro-lens matrix are the images of the light spot on the retina. Yet this is true only when the coordinates of the spots are determined against the maximal intensity dot. At this time, in practically all such instruments, a different algorithm is used (see the Liang et all prototype) where coordinates of the spots are determined by calculating their mass centers (according to their intensity). This method allows to calculate the location of the spot with a precision exceeding the distance between the light-sensitive elements of matrix light detectors such, for instance, as those used in CCD cameras. Moreover, this precision increases as the area of the spot on the light detector increases. So when such algorithms are used, there is no need to make the light spot on the retina small. On the contrary, its excessive smallness leads to the loss of precision.
The same invention suggests that a cylindrical telescope be used to introduce astigmatic correction. The disadvantages of such system include their functional limitation because cylindrical instead of astigmatic wavefront is produced. There are also their complicated mechanical controls. The explicit use of astigmatic instead of cylindrical corrector is far more convenient. Astigmatism may be transformed into a cylinder by adding defocusing, that is, compensating the curvature of one of the cross-sections of the saddle. As is known, defocusing and astigmatism are orthogonal functions while defocusing and cylinder are not. This is why an expansion by such functions of the shape of a wavefront is univalent. Accordingly, if an execution unit, that is, an astigmatic corrector implements these functions, it makes the automatic control of such a device much simpler.
There is a device for compensating astigmatism (see L. S. Urmakher and L. I. Aizenshtat, Ophthalmologic Instruments, 1988, p. 288) that includes two systems rotating around the optical axis, cylindrical or toric lenses of different signs and mechanically engaged manual controls. A disadvantage of this astigmatism correcting device is that it may not be controlled automatically because its mechanical drive is too complicated, the rotation of the said lenses being mutually dependent (their turning angles are equal, while the axis of the instrument is adjusted by turning the whole device around its optical axis).
Because there is no test pattern projector in this prototype, the device is not suitable for testing visual acuity and for subjectively assessing the quality of and controlling the correction.
Another disadvantage of the instrument is the lack in it of a self-testing capability and of the automatic calibration of its mechanical and optical elements in order for the instrument to remain in a usable condition.
During the use, the moving mechanical elements of aberrometers may go out of their working positions. Moreover, the adaptive elements (mirrors) are generally non-linear. For instance, in piezo-controlled mirrors, non-linearity and hysteresis reach 25% of the control range. This is why the results of the use of control signals for estimating correction profiles become incorrect. The known aberrometers are calibrated with optical test elements. These are either external systems optically analogous to the eye or similar devices built into the optical system of the instrument and using beam-splitting plates (U.S. Pat. No. 6,637,884). External calibrating systems require precise placing and certain skills on the part of operators. The need for operators' participation makes automation impossible. Internal calibrating devices that include optical testing elements are hard to make and require additional beam-splitting plates or switchable optical elements.
The lack in the existing instrument of an adjustment system used for aligning the input pupil of the instrument with the pupil of the eye reduces the precision of measurements, the drawback becoming greater as the order of measured aberrations becomes higher and their magnitude increases.
There are various devises used for adjusting required distances between the ophthalmologic instrument and the eye.
There is a device (U.S. Pat. No. 4,881,807) where the distance between the instrument and the eye is determined by a computer estimating the positions of Purkinje images. The use of a computer makes the instrument and the adjustment procedure significantly more complicated. The problem is the difficulty, for the operator, of finding the working area of the electronic system and determining the direction of misalignment.
The prototype ophthalmologic device used for measuring and operating on patients' eyes (U.S. Pat. No. 5,562,656) is the closest, as concerns the design and purpose, to the substance of this application, that is, the device for adjusting the aberrometer and the aligning method implemented in it.
The existing device includes a light source, a system for projecting the images of marks to a patient's eye, a system for visually controlling the positions of the mark images as related to each other, and a system for three-dimensional positioning of the instrument in relation to the eye. The projecting system includes two projectors placed at an angle to the optical axis of the device creating the images of one or several slots on the cornea. Watching, through a microscope, the positions of these images in relation to each other, one may estimate the distance between the instrument and the eye.
The adjustment method implemented in this device, which is used for measuring and operating, includes illuminating the eye, projecting slot images onto it, visually controlling the positions of these projected slot images and the three-dimensional positioning of the instrument.
The disadvantages of this device and adjustment method include the low precision of measurements because it depends on the forming of the slot images on the cornea and also the limitation of its use, when the light source is weak, by the visible part of the light spectrum which is inconvenient for the patient whose cornea is transparent so observation is possible only because of light scattering. In the infrared zone, the scattered slot images will not be visible on the cornea due to a low contrast. Besides, the design of the marks used in the prototype device does not allow the operator to explicitly determine the direction of misalignment.